It is known that the use of a uniform bias charge, or FAT ZERO, in a CCD shift register improves the transfer efficiency of the shift register, see "Charge Transfer Devices" by Sequin and Tompsett, Academic Press, Inc., 1975, page 73. To insure that a high signal-to-noise ratio is achieved in the CCD, the magnitude of the FAT ZERO must be carefully controlled.
One way of introducing a controlled amount of charge into the potential wells of a CCD is by the "fill and spill" method, see Sequin and Tompsett, page 49. According to the "fill and spill" method, an input signal voltage is applied as a voltage difference between two electrodes in a CCD, to produce a metering well under the second electrode. The size of the metering well depends linearly upon the signal voltage difference. An input diode is then pulsed to over fill the metering well with signal charge and to subsequently drain the excess charge from the metering well to the level defined by the first electrode.
This fill and spill method provides generally good linearity, and reasonably low noise. However, in some situations where very small, very accurate charges are required, it proves to be difficult to achieve reliable control of the charge size and uniformity. An example of such a situation is the image sensor shown in U.S. Pat. No. 4,521,106 issued June 4, 1985 to the present inventor. The image sensor is employed in a rangefinder device to detect the apparent location of a spot of light projected on an object in a scene. The distance measuring accuracy of the device is a function of the signal-to-noise ratio that can be achieved in the image sensor. The image sensor includes a row of photodiodes charge coupled on one side by a transfer gate to a drain diode for resetting the photodiodes, and charge coupled on the other side by a second transfer gate to a CCD shift register, for reading out the photocharges generated by the photodiodes. The channels under the transfer gate are surface channels, and the CCD shift register is a buried channel. The signal-to-noise ratio of the image sensor is improved if a FAT ZERO is introduced into the CCD shift register and especially into the surface channel between the photodiodes and the CCD shift register. The photodiodes in the image sensor can be employed as metering wells to introduce bias charges into the potential wells of the CCD by direct analogy to the conventional "fill and spill" process. However, to achieve a high signal-to-noise ratio, very small, accurately metered charges need to be used. It is difficult to achieve this high level of accuracy with the conventional spill and fill technique, because of the difficulty of generating carefully controlled differences in potential barrier heights between the two transfer gates.
Accordingly, it is an object of the present invention to provide an improved fill and spill method and apparatus for introducing small signals into the charge transfer cells of a CCD that is free from the difficulties noted above.